Fire-avoiding grinding and classifying system and process



J. W. AYERS May 22, 1951 FIRE-AV0ID1NG GRINDING AND CLASSIFYING SYSTEM AND PROCESS Filed July 12, 1946 4770 /VEVS Patentecl May 22, 1951 FIRE-AVOIDING GRINDING AD 'CLASSIFY- ING SYSTEM AND PROCESS Joseph 'W. Ayers, Easton, Pa., assignor to Agrashell, Inc., a corporation of Delaware Application July 12, 1946, Serial No. 683,227

Claims.

This invention relates to process and apparatus for the grinding and classifying of combustible materials and more particularly to the production of iinely ground combustible material by a process including grading as to particle size in a centrifugal air separator system wherein danger of iires, explosions or deterioration due to oxidation of the ground material during the classiiioation and as well during the grinding and the conveying thereof is avoided.

In my co-pending application Serial No. 683,768 led July 15, 1946, I described and claimed a process for producing fine particle size powders and industrial iiours wherein natural organic combustible material is ground in a hot operating grinding mill in a superatmospheric pressure of steam generated by vaporization of water in the mill, which vaporization serves not only to take up the excess heat generated by friction in the mill but also to provide steam in an unfailing amount which positively excludes oxygen from contact with the ground material. The process of the present invention, in its preferred embodiment, includes the process of the said application as its initial step. In its broader aspect the invention is independent of any specic procedure for grinding the combustible material so long as particles of considerable iineness of about 160 mesh or smaller are produced under conditions requiring exclusion of oxygen from the ground material until cooling has occurred.

Finely ground combustible material discharged from conventional grinding mills ordinarily requires classification to obtain products of more uniform size to meet speciiications for the various industrial uses. A typical screen analysis of a iinely divided combustible material produced by or utilized within the process of the invention is as follows:

i; t' i s' Pevgvcnlgtby ar 1c e ize eig Mesh Slze in Microns Through Screen The classification of material of a small mesh, usually smaller than 100 mesh, may be more eiciently accomplished in centrifugal air separators than on screens. In the table below there are described typical air separators contemplated for the practice of the classification process of the present invention and their feed rates as applied to ground walnut shell ilour of the size distribution described above.

Size of Volu- Rate of Cir- A i S(e]p)arator ICnetric culltlion Pvlilsle Flieg; /gtetn lamapacwi in eter) ity Separator Separator Separatol Inches of Feet Cu. ft C. f. lm. water 8 400 10, 000 8 6, OOO-S, 000 6 181 8, 000 9 3, 000-5, 000 4 58 3, 000 7 800-2, 000

At the feed rates described in the above table the suspended dust concentrations in the separators are very high and are in the range of explosive dust mixtures. When explosions have occurred in equipment of this kind serious damage has resulted because of the large cubic capacity Within the separators and the large quantity of dust contained therein. Since iine grinding operations ordinarily produce very hot material for classification in the separators, the separation and classification of the mill discharge is difficult and hazardous due to the danger of explosion of the hot dusts.

An object of the present invention is to provide a system for conveying and classifying nely divided combustible material to and in centrifugal air separators in which danger of fires, explosions or oxygen deterioration is avoided. It is also an object of the invention to provide a process for grinding and classifying natural organic or vegetable material wherein the said operations are economically carried out and wherein the heat of the grinding operation is employed to advantage in the subsequent conveying and classifying steps, all without the above mentioned dangers.

Broadly considered, the present invention may be said to involve an improved process of classifying iinely divided combustible material in centrifugal air classifier systems wherein the ground material is conveyed and classied in an atmosphere of at least relative humidity and such atmosphere is maintained at a temperature above its dew point. In a more limited embodiment, the process includes a grinding operation carried out under a superatmospheric pressure of steam wherein the hot ground material together with steam employed or generated in the mill are passed together through a centrifugal air classifier system wherein the atmosphere is at a relative humidity of at least 85% and the temperature is maintained above the dew point as a result of heat developed in the grinding operation.

Experimentation has demonstrated that if the relative humidity of the atmosphere in the classifying system is maintained as high as the abovementioned 853%-, explosions willnotoccur regardless of the dust concentration in the atmosphere.

In a specific embodiment of the invention,` English walnut shells in their normal air-dry condition containing 8 to 19% moisturawere fedi to a grinding mill and ground thereiniin accordi ance with the process disclosed in the hereinbe.- fore mentioned co-pending application f The. moisture content is generally reduced during the grinding operation to 2 to 4%. andrthe; hotdryi ground product obtained hence is in substantially, dry form and may contain from 2 to 10% mois'- tu're. By operating thez mill at a temperature of approximately 275 F. not only were conditionsA desirable for the satisfactory grindingof the' shells maintainedY but also enough heat was provided in the material discharged from the mill to maintain' a temperatureiny the conveyors and separators, in which the mill" discharge was classied, highV enough to prevent the precipitation ofv moisture from the humid. atmosphere. Thesteam present during the grindingvoperation and that' present' in the classifying systemwere maintained inproper thermal balance tov leeep the whole system under non-explosive conditions` Although the preferredembodimen't of the invent'onf relies only upon theheat generated by the frietionV4 in the mill for supplying the heat energy in' theiwholeA grinding and classifying system, heat may be' supplied, in' aA less desirable embodiment, from' someY external source-` It might be necesi sary in` certain instances that thernill be run at al lower temperaturev than that whichk provides sufcient heat for th'efsystem and under suchconditions, heat may be supplied to the system by feeding superheated steam to the separators or by externally heating' the system therebyl to keep the atmosphere therein above the dew point.

For accomplishing the results of the presentinvention the grinders, conveyers and separators must'be provided with the necessary seals in order to prevent excess radiation loss, dust loss and loss of steamv necessary to hold the relative humidity o'fthe atmosphere in the separatorl at 85% or above. In any ordinary apparatus, the ground product, as itpasses through conveyers, elevators, and separators loses heat and its temperature drops, but in the preferred process of the present invention,l the initial temperature of the ground material. and' steam discharged therewith is sufciently high to prevent precipitation of water anywhereV inY the' classifying. system. When the grinding' mill is run ata temperature of 275 Fl, the' separator temperature will be maintained in the neighborhood of 180 to 212 F.

Inn accordance with a still further embodiment of the present invention, a high feed rate tothe separators' is maintained by recirculating a sub- 'avoid' a; cooling'sufcient toY permit the dew' point.

or condensation to be reached. A small recirculating load does not have suiiicient heat capacity to hold the temperatures of the separators above the dew point of the atmosphere therein because of radiation loss in the system and also the rate of production of the nnished product is low, and therefore a large recirculating load ratio of at least'3 to l1, ifs; employed in accordance,v with this embodiment. That is to say the recirculating load in the system is at least 3 times the weight of the finished product collected from it.

Radiat-iony loss from the conveyers and separatorsimay be lessened by the employment of insulation around the apparatus or around the entire system buttheecost of installing such insulation and maintaining it is prohibitively high. The preferred. embodiment of the present invention as hereinbefore described involving the hot grinding' opera-tion and preferably also a high recirculating load of material, is economical and more practical for maintaining sufficiently highy temperatures.

In order that the. invention may be:r more. com.- pletely understood, a schematicA drawing; is ineluded' showing in Figure. 1.a grinding, conveying and-"classifying system. in which the` present invention may be practiced. A vertical section. of a suitable type grinding: mill is shownv in Fig.- ure 2f.

With reference to the' drawing, there is illus;-r tr'ated' af'eed storage hopper, Ill for/the. raw material'. conn'ected toi a,V grinding mill I l@ throngha screw .conveyer i2 and a rotaryu feed and seal valvey 1.3. The mill ll: is of ai conventional type having oppositely rotating grinding plates I4 and I-5`. Water' serving: to, remove excess heat from the mill through Vaporization and serving'to` pro;- v-ide': a superatmosp'h'ere: of steam is introduced into the mill through the valved conduit I6.

The milly is. provided with an outlet I1 at its bottom forV the ground material and is connected to`r a conveyer I8 having therein an interrupted screw i9 adapted to provide a plug seal of the ground material in the usual manner' in thefsnace ZYI therebetween. In the feedr end or the screw conveyer a vent pipe. 22 having a regulatablervalve or damper 23 is provided for maintaining a. pres.- sure of 1 to 3 pounds per square inch or moreand for relieving any excess vapor pressure which may' develop in the mill.

The conveyer I8, at its discharge end, is connected to an elevator 2e which at its top end connects to a plug seal conveyer 25 connected in turn at its discharge end to the primary air classifier 26.

The discharge opening on this air c1assiler'26 for the coarse material is connected to a plug seal' conveyer 2l' and to spouts 28 andl 29. The spout 28 connects through a gate valve 3l to a secondary classifier 32. The spout 29 is connected with another spouty 33 joined to the outlet for coarse; material from the air classier 32 and is also joined tothe screw conveyer l2 leading to the grinding mill l I. The ne material classified in the classifier 32 passes to a receiver St from which it isy discharged into shipping containers.

The primary air classier 26 is connected at its outlet for finely ground material to a plug seal conveyer 35 which at its discharge end is connected to a secondary air classier 3S.. The outlet on. the air classier 36 vfor the coarser material is: connected to a plug seal c-onveyer 3l in turn connected to the receiver 3S. The outlet for the finest Vmaterial from the said classier is connected through a plug seal conveyer 39 tothe receiver 4I. From the receivers 38 and 4I the respective materials are introduced into shipping containers. For introducing additional steam to maintain the required temperature in the system, valved steam inlets 3l! are provided on the classifiers.

From the foregoing description it will be noticed that the grinding mill Il and the air classifiers 26 and 36 are surrounded by seals such that the hereinbefore described vapor pressures may be maintained and oxygen may be excluded. Air is excluded from the classier 32 by means of seals created by the ground material in the conveyers 21 and I2, the feed valve I3 and in the outlet of the receiver 34 the latter beingprovided with a conventional sleeve or sock 43 attached to the spout 34 and adapted to hang over hook All when not discharging.

In any typical operation of the process of the present invention wherein the product described in the table set out herein is produced, the finest ground material withdrawn from the receiver 4i is of a size which will pass through a 325 mesh screen. The next larger cut is that obtained in the receiver 38, such material being that which is between about 200 and 325 mesh. The third and coarsest cut removed from the system is that obtained in the receiver 34 and such is of a size which will pass through a 100 mesh screen but not through a 200 mesh screen` The ground material shown in the drawing as being returned through the spout 33 to the mill I I may, in an alternative procedure, be separated from the system and, if so desired, further classied for sale as various large size cuts.

The hereinbefore described apparatus may be provided with any additionally desired valves and seals and with the usual magnetic separator to take out stray particles of metal. Such separator is shown diagrammatically at 42.

It should be understood that the present invention is not limited to the specific procedures, conditions and apparatus herein described but that it extends to all equivalents which will occur to those skilled in the art upon consideration of the scope of the claims appended hereto.

I claim:

l. In the classification of finely divided explosive dusts of combustible material in a centrifugal air separator system, the process of avoiding lires, explosions and oxygen deterioration which comprises, conveying and classifying the material in an atmosphere of at least 85% relative humidity and at a te-mperature above its dew point and returning at least 3A of the ground material from the separator again through said separator whereby a high feed rate to the separator is maintained and the heat of the returned material serves to maintain the atsmosphere in the separator at a temperature above its dew point.

2. A grinding and classifying system for combustible materials all of the elements'of which are enclosed and sealed for the exclusion of air, comprising in combination, a hot-operating, drygrinding mill of a construction capable of grinding the material to particles of explosive dust size, sealed feeding and discharge means connected thereto for the materials, inlet means for introducing H2O into the mill, a conduit connecting the mill with a classier for transferring the ground material, a centrifugal air classier connected to said conduit through a sealed feeding means, separate sealed discharge means for the fine and coarse cuts connected to said classifier, and a conduit connecting the discharge means for the coarse cut to the mill through a sealed feeding means.

3. In a grinding and classifying system for combustible materials the combination which comprises, an enclosed dry-grinding mill of a type adapted to operate at temperatures above 212 F. and to grind material to particles of explosive dust size, an enclosed centrifugal air classifier, sealed feeding -means connected to the mill for introducing H2O and the material to be ground, a sealed discharge means connected to the mill, an enclosed conduit connecting the mill to the classier for transferring the ground discharge, sealed feeding means in said conduit for introducing the material into said .classien a sealed discharge means connected to the classifier for the finer out, a sealed discharge means connected to the classifier for the coarser cut, an enclosed conduit connecting said last mentioned discharge means with said mill through a sealed feeding means.

4. A grinding and classifying system for combustible materials, all of the elements of which are enclosed and sealed for the exclusion of air, comprising in combination, a hot-operating, drygrinding mill of a construction capable of grinding the material to particles of explosive dust size, sealed feeding means connected thereto for the yn'iaterials to be ground, inlet means for introducing In() into the mill, sealed discharge means connected to the mill for the materials and steam, a conduit connecting the discharge means with a classifier for transferring the ground material and water vapor, a centrifugal air classifier connected to said conduit through a sealed feeding means for said material and water vapor, sealed discharge means connected to said classifier for the nely ground material, a second sealed discharge means connected to said classifier for the coarser ground material and water vapor and conveying means connected to the second discharge means for transferring the coarse cut and water vapor to the mill through a sealed feeding means.

5. A grinding and classifying system for combustible materials, all of the elements of which are enclosed and sealed for the exclusion of air, comprising in combination, a hot-operating, drygrinding inill of a construction capable of grinding the material to particles of explosive dust size, sealed feeding and discharge means connected thereto for said materials, inlet means for introducing H2O into the mill, a conduit connecting the mill to a classifier for transferring the ground material and water vapor, a centrifugal air classiier connected to said conduit through a sealed feeding means, separate sealed discharge means connected to said classifier for the fine and coarse cuts respectively, a conduit connecting the discharge means for the coarse cut to a second centrifugal-air classifier, separate discharge means for the fine and coarse cuts connected to said second classifier, and a conduit connecting` the last mentioned discharge means to the mill through a sealed feeding means.

6. A grinding and classifying system for oombustible materials, all of the elements of which are enclosed for the exclusion of air, comprising in combination, a hot-operating, drygrinding mill, sealed feeding means connected to the mill'for introducing H2O and the material to be ground, sealed discharge means connectved to the mill for the ground materials and steam, a conduit connecting said discharge means to a classifier for transferring the ground material and water vapor, a centrifugal air classifier connected to said conduit through a sealed yfeeding means for said material and water vapor,

sealed discharge means connected to said classifier for the fine cut of material, sealed discharge means connected to said classifier for the coarser cut and water vapor, a second centrifugal air classifier connected to said last mentioned discharge means, a discharge means connected to said second air classifier for the finer cut and a discharge conduit connecting said second classier to said mill through a sealed feeding means for returning the coarser cut and water vapor to said mill.

7. The process of avoiding fires, explosions and oxygen deterioration in the grinding and classifying of combustible materials which comprises, grinding such material to particles of explosive dust size in a superatmospheric pressure of steam, passing the hot ground material together with the steam through a centrifugal air separator wherein the ground material is separated from the atmosphere therein, and maintaining the atmosphere in the centrifugal air separator at a relative humidity of at least 85% and at a temperature above the dew point of the atmosphere by means of the heat developed in the grinding operation supplemented by additional heat sup-- plied to the separator.

8. The process of avoiding fires, explosions and oxygen deterioration in the grinding and classifying of combustible materials which comprises, grinding such material to particles of explosive dust size in a superatmospheric pressure of steam, passing the hot ground material together with the generated steam through a centrifugal separator wherein the ground material is separated from the atmosphere therein, and maintaining the atmosphere in the separator at a relative humidity of at least 85% and at a temperature above the dew point by means of the heat developed in the grinding operation supplemented by superheated steam fed into the separator.

9. In the classification of hot explosive dusts of combustible materials containing about 2 to 10% moisture in a centrifugal classifying system having sealed inlets and outlets thereto, the process of avoiding res, explosions and oxygen deterioration which comprises continuously conveying and classifying the material in said system in a steam-containing atmosphere maintained at a relative humidity of at least 85%, and maintaining said atmosphere at a temperature above its dew point whereby air is positively excluded from said system.

10. The process of avoiding fires, explosions and oxygen deterioration in the grinding and classifying of combustible material which comprises, introducing such material into a mill in air-dry condition, grinding such material to a powder of explosive dust particle size in a superatmospheric pressure of steam, and passing the hot ground material together with the steam through a centrifugal classifier, and maintaining the atmosphere therein at a relative humidity of at least 85% and the temperature above the dew point by means of heat developed in the grinding operation, whereby there is obtained a ground powder containing no greater amount of moisture than that initially present in the material before grinding.

11. The process of avoiding fires, explosions and oxygen deterioration in the grinding and 4,classifying of combustible material which com- '8 prises, introducing such material into a millin air-dry condition, grinding such material to a powder of explosive dust particle size in a superatmospheric pressure of steam, and passing the hot ground material together with the steam through a centrifugal classifier, and maintaining the atmosphere therein at a relative humidity of at least and the temperature ab-ove the dew point by means of heat developed in the grinding operation, whereby there is obtained a ground powder containing no greater amount of moisture than Ythat initially presentV in the material before grinding, and generating steam in said mill in an amount sufficient to maintain the herein specified humidity and temperature in the atmosphere in the said centrifugal classifier.

12. The process of avoiding lires, explosions and oxygen deterioration in the grinding and classifying of combustible material which comprises, introducing such material into a mill in air-dry condition, grinding such material to a powder of explosive dust particle size in a superatmospheric pressure of steam, and passing the hot ground material together with the steam through a centrifugal classifier, and maintaining the atmosphere therein at a relative humidity of at least 85% and the temperature above the dew point by means of heat developed in the grinding operation, whereby there is obtained a ground powder containing no greater amount of moisture than that initially present in the material before grinding, and venting off a portion of the steam provided in said mill during its passage from the mill to the centrifugal classiner.

13. The process of avoiding fires, explosions and oxygen deterioration in the grinding and classifying of combustible material which coinprises, introducing such material into a mill in air-dry condition, grinding such material to a powder of explosive dust particle size in a superatmospheric pressure of steam, and passing the hot ground material together with the steam through a centrifugal classifier, and maintaining the atmosphere therein at a relative humidity of at least 85% and the temperature above the dew point by means of heat developed in the grinding operation, whereby there is obtained a ground powder containing no greater amount of moisture than that initially present in the material before grinding, and generating the steam employed by the mill by vaporizing the same by means of the heat developed in the grinding operation.

14. In the grinding and classification of finely divided explosive dusts of combustible material in a mill and centrifugal separator and classiiier system, the process of avoiding fires, explosions and oxygen deterioration which comprises grinding such material to a powder of explosive dust particle size in a superatmospheric pressure of steam, passing thehot ground material together with the steam through a centrifugal separator and classifier system lwherein the ground material is classied in and separated from the atmosphere therein in an atmosphere of at least 85% relative humidity and at a temperature above its dew point and returning at least three-fourths of the ground material from the separator and classier system again through said mill and thence through said separator and classifier system whereby a high feed rate to this system is maintained and the heat of the returned material serves to retain the atmosphere in the system at a temperature above its dewllpoint. i

15. In the grinding and classication of finely divided explosive dusts of combustible material in a mill and centrifugal separator and classier system, the process of avoiding res, explosions and oxygen deterioration which comprises grinding such material to a powder of explosive dust particle size in a superatmospheric pressure of steam, passing the hot ground material together with the steam through a centrifugal separator and classifier system wherein the ground material is classied into a coarse fraction and a ne fraction and separated from the atmosphere therein, conveying the said ne fraction through a second separator-classier system wherein a secondne and coarse fraction is made and separated therefrom, returning. at least threefourths of the coarse ground material again through said mill and thence through said separator and classifier systems whereby a high feed rate to the systems is maintained and the heat of the returned material serves to maintain the atmosphere in the systems at a temperature above its dew point, and conveying, classifying and separating the material throughout said systems in an atmosphere of at 1 ast85 relative humidity and at a temperature above its dew point.

JOSEPH W. AYERS.

REFERENCES CITED The following references are' of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 448,516 Gardner Mar. 17, 1891 479,188 Krestner July 19, 1892 1,367,895 Schouten Feb. 8, 1921 1,385,674 Fraser July 26, 1921 1,406,938 Cunningham Feb. 14, 1922 1,478,205 Davis Dec. 18, 1923 1,708,123 Day April 9, 1929 1,795,603 Hussey Mar. 10, 1931 2,008,892 Asplund July 23, 1935 2,062,064 Knox Nov. 24, 1936 2,100,691 Foster Nov. 30, 1937 

